Abstract
: ObjectiveThis article describes a first attempt to generate a standardized and safe selective surface electrostimulation (SES) protocol, including detailed instructions on electrode placement and stimulation parameter choice to obtain a selective stimulation of the denervated zygomaticus muscle (ZYG), without unwanted simultaneous activation of other ipsilateral or contralateral facial muscles. Methods: Single pulse stimulation with biphasic triangular and rectangular waveforms and pulse widths (PW) of 1000, 500, 250, 100, 50, 25, 15, 10, 5, 2, 1 ms, at increasing amplitudes between 0.1 and 20 mA was performed. Stimulations delivered in trains were assessed at a PW of 50 ms only. The stimulation was considered successful exclusively if it drew the ipsilateral corner of the mouth upwards and outwards, without the simultaneous activation of other ipsilateral or contralateral facial muscles. I/t curves, accommodation quotient, rheobase, and chronaxie were regularly assessed over 1-year follow-up. Results: 5 facial paralysis patients were assessed. Selective ZYG response in absence of discomfort and unselective contraction of other facial muscle was reproducibly obtained for all the assessed patients. The most effective results with single pulses were observed with PW ≥ 50 ms. The required amplitude was remarkably lower (≤5 mA vs. up to 15 mA) in freshly diagnosed (≤3 months) than in long-term facial paralysis patients (>5 years). Triangular was more effective than rectangular waveform, mostly because of the lower discomfort threshold of the latter. Delivery of trains of stimulation showed similar results to the single pulse setting, though lower amplitudes were necessary to achieve the selective ZYG response. Initial reinnervation signs could be detected effectively by needle-electromyography (n-EMG). Conclusion: It is possible to define stimulation parameters able to elicit an effective selective stimulation of a specific facial muscle, in our case, of the ZYG, without causing discomfort to the patient and without causing unwanted unspecific reactions of other ipsilateral and/or contralateral facial muscles. We observed that the SES success is strongly conditioned by the correct electrode placement, which ideally should exclusively interest the area of the target muscles and its immediate proximity.
Highlights
Facial paralysis is a neurological condition in which the facial nerve functionality is completely lost, i.e., the innervated facial muscles are completely denervated, in contrast to facial paresis, a condition in which some motor activity can still be detected
A major issue against a widespread use of surface electrostimulation (SES) in the treatment of facial paralysis is the difficulty to determine a combination of parameters capable to generate a selective stimulation delivered by surface electrodes
Our results suggest that the use of rectangular waveform, of pulse widths (PW) < 50 ms, and/or the use of train rather than single pulse stimulation are all factors that may enhance the risk to reach the discomfort threshold and/or cause unspecific activation of other facial muscles or co-contractions before an amplitude could be applied that elicited a selective zygomaticus muscle (ZYG) response
Summary
Facial paralysis is a neurological condition in which the facial nerve (cranial nerve VII) functionality is completely lost, i.e., the innervated facial muscles are completely denervated, in contrast to facial paresis, a condition in which some motor activity can still be detected. This article focuses exclusively on the peripheral facial palsy, a form affecting the ipsilateral mimic muscles, and, in particular, the eyelids, the forehead, and the mouth. Previous studies on the therapeutic use of electrostimulation [21,22] have provided initial evidence that SES can effectively counteract atrophy, suggesting that it has the potentiality to slow down if not even to prevent the reduction of muscle volume and functionality in patients suffering from facial paralysis [21,22,23,24,25,26]. A major issue against a widespread use of SES in the treatment of facial paralysis is the difficulty to determine a combination of parameters capable to generate a selective stimulation delivered by surface electrodes. The electrode placement could be corrected whenever an unspecific response of other facial muscles is observed
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